延安气田下二叠统山西组流体包裹体特征及其地质意义

Characteristics and geological significance of fluid inclusions in the Lower Permian Shanxi Formation in the Yan’an Gas Field

近年来,鄂尔多斯盆地东南部上古生界天然气勘探取得了重大突破。为了明晰该区上古生界天然气的成藏过程,以区内延安气田上古生界气藏主力产层--下二叠统山西组为例,通过对储层流体包裹体颜色、形态、成分、均一温度等特征的分析,结合埋藏史、热史模拟,确定了包裹体形成期次,厘定了烃类充注的地质年代,分析了油气充注与成岩演化的耦合关系。研究结果表明:①延安气田山西组储层流体包裹体主要赋存于自生石英、碳酸盐胶结物及石英颗粒愈合缝中,有CO2、烃类和盐水3种类型的包裹体。②烃类包裹体的形成主要有两期,其中第一期主要发育在石英次生加大边和石英颗粒愈合缝中,均一温度介于90~110℃,气态烃组分以富CO2和CH4为主;第二期主要发育在石英加大边、碳酸盐胶结物及石英颗粒愈合缝中,均一温度介于130~160℃,气态烃组分以富CH4为主。③山西组存在着两期烃类充注,其中第一期发生在晚三叠世-早、中侏罗世,该时期有机质开始生排烃并充注,长石和岩屑溶蚀形成次生孔隙,化学压溶作用开始发生并形成少量石英加大边;中、晚侏罗世,烃源岩进入成熟阶段,开始大量生排烃并开始第二期烃类充注,化学压溶作用加强,石英胶结大量发育;至早白垩世,烃源岩热演化达到高-过成熟阶段,大量生气,石英胶结继续发育,铁白云石发生沉淀;早白垩世末至今,烃源岩生烃作用逐渐停止。

In recent years,great breakthroughs have been made in natural gas exploration in Upper Paleozoic in the southeastern Ordos Basin.In order to understand the gas accumulation process of Upper Paleozoic in this area,this paper analyzed the characteristics of reservoir fluid inclusions(e.g.color,form,composition and homogenization temperature)by taking the principal pay zone of Upper Paleozoic gas reservoir in the Yan’an Gas Field of southeastern Ordos Basin as an example.Then,combined with the burial history and thermal history simulation,the formation period of fluid inclusions was determined,and the geological age of the hydrocarbon charging was defined,and the coupling relationship between hydrocarbon charging and diagenetic evolution was analyzed.And the research conclusions were as follows.First,the reservoir fluid inclusions of Shanxi Formation in the Yan’an Gas Field mainly exist in authigenic quartzs,carbonate cements and concrescence fractures of quartz grain,and they are classified into three types,i.e.,CO2 inclusion,hydrocarbon inclusion and brine inclusions.Second,the formation of hydrocarbon inclusions is mainly divided into two phases.In the first phase,they are mainly developed at quartz overgrowth edges and concrescence fractures of quartz grain,where the homogenization temperature is between 90℃and 110℃and the gas hydrocarbon components are mainly rich in CO2 and CH4.And in the second phase,they are mainly developed at quartz overgrowth edges,carbonate cements and concrescence fractures of quartz grain,where the homogenization temperature is between 130℃and 160℃,and the gas hydrocarbon component is rich in CH4.Third,there are two hydrocarbon charging phases in the Shanxi Formation.The first phase occurred in the Late Triassic to the Early and Middle Jurassic,during which the organic matter began to generate,expel and charge hydrocarbon,secondary pores were formed by the dissolution of feldspar and lithic,and chemical pressure dissolution initially occurred and generated a small number of quartz overgrowth edges.In the Middle and Late Jurassic,source rocks entered the mature stage and began to generate and expel a great amount of hydrocarbon;the second phase of hydrocarbon charging began;the chemical pressure dissolution was strengthened;and the quartz cement was developed in a great quantity.In the Early Cretaceous,source rocks reached the high and over-mature stage and generated a large amount of gas,quartz cements continued to grow,and ferroan dolomite began to precipitate.Since the end of the Early Cretaceous,hydrocarbon generation from source rocks gradually ceased.